The separation of particles from a fluid may have applications in both the clinical diagnostic and the basic research fields. For a number of applications, separation is performed by applying differential forces to the positive fraction (e.g. cells of interest) and the negative fraction (e.g. background cells). Devices have been described where differences in physical properties have been used to separate specific cells or molecules from solutions of mixed population. These physical properties have included size, motility, electric charge, electric dipole moment, optical qualities, and magnetic susceptibility. Another approach has been to separate cells based on binding of specific surface markers. For example, surfaces of microfluidic channels have been patterned with a variety of antigen capture molecules; a subset of the cell population then interacts with the surface and gets immobilized by binding the surface antigen.
Another approach taken has been to selectively bind beads of a paramagnetic material to the cells of interest, typically via a surface marker present at the cell membrane. The positive fraction is then separated using a magnetic field gradient by either placing a magnet close to the cell suspension or microfluidic channel, or by using an external magnet in order to magnetize structures that have be incorporated in the microscale device thereby amplifying the field gradient in an adjacent region of space. Alternately, the negative fraction can be actively separated, leaving the positive fraction. Various macroscale and microscale devices have been presented in order to separate magnetically labeled species.